John L. Cleveland, Instructor

1. Chapter 8: Arrays John L. Cleveland, Instructor

2. Arrays 1 • Group of Variables (called elements) that have the same type. • To refer to an array, we refer to the array name and position of the element. • Every array begins at the zero element. • Position number in parenthesis is called an index or subscript. • Array number much be a non-negative integer, • An indexed array name is an lvalue, and can be used on left side of an assignment statement to place a new value into an array.

3. Arrays 2 • In VB, every array “knows” its own length. • Including the Length property, all arrays have access to methods and properties of the System.Array class. • The last element in an array can be fetched with GetUpperBounds.

4. Declaring and Allocating Arrays • To declare and array, you provide array’s name and type, Dim C as Integer(). • The declaration creates the a variable but stores no value in memory. • Arrays can be declared to contain any type. • Before using an array, specify size and allocate memory. • The initializer list in brackets {} specifies initial values which can be a comma-separated list. • When the initializer list is empty, the elements are initialized to their default values.

5. Using Arrays • All elements in an array must be of the same type. • VB provides mechanisms that prevent accessing elements outside the bounds of the array.

6. Passing an Array to a Method • Pass an array name to a methods with no parenthesis. • Every array element knows its own upper bound so no need to pass the upper bound as a separate argument. • The method’s parameter list must specify that an array will be received • It is normally inappropriate to specify that an array be passed using the keyword ByRef since the are passed by reference by default in VB. • Used the indexed name of the array element as an argument in the method call to pass an array element

7. For Each…Next • Iterate through an array using the For Each…Next repetition statement. • When used with a one-dimensional array, for each…next iterates through the range of values from the zero element through to the value returned by GetUpperBound().

8. Sorting an Array • Sort data in ascending or descending order is one of the most popular computing applications. • Class Array provides methods for creating, modifying, sorting and searching arrays. • Shared method Sort of Class Array takes an array as its argument and sorts in ascending order. • To sort in descending order, first call method Sort and then method Reverse.

9. Searching an Array • Process of locating a particular key element value in an array is called searching. • Linear method works well for small or unsorted arrays, but is inefficient for large unsorted arrays. If the array is unsorted, high-speed binary search can be used. • The Array class provides a method BinarySearch which is used to search a sorted array.

10. Rectangular and Jagged Arrays • One-dimensional arrays contain one row of values. • Multi-dimensional require two or more indices to identify particular elements. • Two types of two-dimensional arrays—rectangular and jagged. • In rectangular arrays, each row is the same size. • Jagged arrays are maintained as arrays of arrays and rows can be of different sizes. • In jagged arrays, the second dimension actually is an index into the one-dimensional index representing the current row. • Use next For…Next to traverse each dimension of multidimensional array.

11. Variable-length Parameter lists • ParamArray used by methods a variable number of arguments. • All arguments passed to the ParamArray must be of the same type. • Otherwise a compilation error occurs when Option Strict is on.

12. ReDim • Use to change array size dynamically at program execution time. • Can change the array size but not the number of elements or type. • ReDim ArrayName(New UpperBound) • The keyword Preserve after ReDim saves the old values and upper bound but if the new array is smaller than old, some values may be lost.

13. ByVal versus ByRef 1 • Reference types like arrays and other objects are actually passed by reference even if the ByVal keyword is used. • That means changes made to the objects in called methods is actually made to the original objects in the callers. • ByRef is another way arguments can be passed. • When ByRef is used, the caller actually gains control over the original reference in the caller. • That means the calling method can actually replace the the caller with a reference to a different object or even Nothing • Such behavior can lead to disastrous results. You always need to keep in mind what values you are changing.

14. ByVal versus ByRef 2 • IsNot operator can compare two referenced objects to see if they are the same. If not, IsNot returns False.

15. Other Terms to Know 1 • Allocate an array with New • Array as an Object • Array Declaration • Array Initialized to zeroes • Array of arrays • Bar Chart • Double subscripted arrays • Exception for invalid array indexing • Explicit Array bounds

16. Other Terms to Know 2 • Ignoring array Element zero • IndexOutofRange Exception • Inner For Statements • Inner Loop • Innermost set of parenthesis • Iterative Binary search • Jagged Array • Key Value in Searching

17. Other terms to Know 3 • M-by-n array • Off-by-one array • Outer For Statement • Subarray • Table • Table Elements • “Walk” past the end of an array • Zero-based Counting

18. Assignment Week 2Arrays in Chapter 8. • Pages 394-395. Self-review exercises 8-1 through 8.2 • Pages 395- 397. Exercises 8.3 through 8.9